1. Field of the Invention
The present invention relates to the use of certain silicone-polyether copolymers as surface-active substance in the production of high resilience (HR) polyurethane foams.
2. Description of the Related Art
According to the prior art, the processes employed in the production of polyurethane foams are differentiated into a number of types. This differentiation may be in respect of both the types of chemicals used and the production machines used. Thus, for example, a distinction is made between batchwise manufacture in boxes or molds and continuous manufacture on various types of foaming machine. Significantly different groups of polyurethane foams art, also differentiated in terms of the raw materials used and thus the types of foam produced.
Thus, for example, rigid foams are, in terms of their property profile and use alone, significantly different from flexible foams, but even within the group of flexible foams a distinction is made between hot-cure foams and high resilience foams. Here, an HR foam is a highly elastic polyurethane foam formed by reaction of at least one bifunctional polyisocyanrate, e.g. tolylene diisocyanate or diphenylmethane diisocyanate, with at least one polyol which has at least two hydroxyl groups per molecule and has, on average, a high proportion of primary hydroxyl groups. Owing to the high content of primary OH groups, the polyols have a high reactivity toward the isocyanates.
In contrast to conventional polyurethane foams, namely hot-cure foams, a high crosslinking density is therefore achieved even during foaming. This has the advantages that input of relatively large amounts of energy during curing is usually unnecessary and that the total time for curing of the foams is reduced. A disadvantage is, however, that the tendency to form closed-celled foams is increased and therefore the processing latitude is also narrowed. Processing latitude refers to the tolerance limits within which it is possible to deviate from a formulation without a danger of forming stable and at the same time sufficiently open-celled foams or foams which can easily be opened by crushing. According to the prior art, this higher reactivity of the foaming components and the resulting narrower (in comparison to hot-cure flexible foam) processing latitude calls for special stabilizers. It does not allow use of products whose structure or molecular weight corresponds to the foam stabilizers which are successfully used in the production of hot-cure foams as foam stabilizers for HR foam production.
This prior art is reflected in a number of different patents in the polyurethane field. For example, as early as 1960, GB-A-907 971 stated that high molecular weight silicone oils cause an excessively large number of closed cells in the foam. For this reason, low molecular weight polydialkylsiloxanes having viscosities of from 2 to 9 centistokes at 25.degree. C. are proposed in the cited patent. These then guarantee, compared to the prior art, an improved processing latitude (see column 2, lines 55 ff.). In the field of HR polyurethane foams, many low molecular weight polydimethylsiloxanes or short polydimethylsiloxanes modified by low-polarity groups have generally been proposed and are also used according to today's state of the art.
DE-A-36 26 297 describes siloxanes containing chloropropyl radicals as HR foam stabilizers. In this case too, the compounds described are relatively low molecular weight siloxanes which are substituted by groups of relatively low polarity. EP-A-0 041 216 also uses siloxanes which all have a short chain length and are additionally substituted by low-polarity and low molecular weight substituents, in this case alkoxy groups having from 1 to 7 carbon atoms. Even in those cases in which conventional silicone polyethers of relatively high molecular weight have hitherto been used in HR foam, e.g. in EP-A-0 043 110, these substances have essentially been used as minor amounts in admixture with typical HR foam stabilizers. For example, in EP-A-0 043 110, the predominant part, at least 65%, of the stabilizer preparation used consists of typical HR foam structures with very short siloxanes with chain lengths of .ltoreq.10 substituted with cyanoalkyl groups or end-capped polyethylene oxide units of low molecular weight.
U.S. Pat. No. 4,477,601 also unambiguously states that the major part of a cold-cure foam stabilizer typically comprises low molecular weight siloxanes having relatively small proportions of polar substituents. Thus, according to the cited patent, at least 78% of stabilizers having a typical HR foam stabilizer structure is necessary. According to the text, proportions greater than 22% of a stabilizer for conventional flexible urethane foams, in this text described as hot-cure flexible foams, result in, a very small processing latitude with a tendency to form closed-celled foams which sometimes shrink.
U.S. Pat. No. 4,119,582 describes a process for producing cold-cure foams using, a foam stabilizer comprising a mixture of a "high" molecular weight siloxane-oxyalkylene copolymer with a "low" molecular weight siloxane-oxyalkylene copolymer. While the "high" molecular weight siloxane-oxyalkylene copolymer has at least five oxyalkylene units in each oxyalkylene block and is present in the mixture in an amount of from 84 to 99.9% by weight, the "low" molecular weight siloxane-oxyalkylene copolymer has up to four oxyalkylene units in each oxyalkylene block and is present in the mixture in an amount of from 0.2 to 16% by weight. The "high" molecular weight siloxane-oxyalkylene copolymer has a molecular weight in the range from 1000 to 16,000, while the "low" molecular weight siloxane-oxyalkylene copolymer covers a molecular weight range from 500 to 10,000. Accordingly, the two classes of copolymer are not necessarily differentiated by their molecular weights but rather by the number of oxyalkylene units in their oxyalkylene blocks.
Nevertheless, even the "high" molecular weight siloxane-oxyalkylene copolymer is a small molecule compared to customary flexible block foam stabilizers. See, for example, U.S. Pat. No. 5,357,018 or U.S. Pat. No. 5,306,737).